This invention relates to the machining of metals, and in particular to an improved method of cooling and lubricating the cutting zone during machining.
The machining of metals, such as with a lathe, generates significant amounts of heat which impairs the cutting action of the tool. It was recognized early on that cooling the metal during cutting would improve cutting and extend tool life. With the high speed machining techniques now in use, some type of cooling is virtually essential. This cooling is typically accomplished by a method known as "overhead flooding" in which a relatively large quantity of cooling fluid is flooded over the cooling zone at relatively low speed and pressure. Often the cooling fluid simply flows under gravity.
Lubricants, such as mineral oils, are usually added to the cooling fluid to reduce friction in the cutting zone; however, these added lubricants make disposal of the used cooling fluid more difficult. With anywhere from 2.5 to 5 gallons of coolant liquid used per pound of metal removed, this disposal can pose a difficult problem.
In the early 1950's it was discovered that applying water under low to moderate pressure to the cutting zone improved cutting and extended tool life. An article by Piggott and Collwell, entitled "Hi-jet System for Increasing Tool Life," 6 SAE Quarterly Transactions 547 (1952), incorporated herein by reference, disclosed that injecting a stream of coolant such as water under moderate pressure of about 400 p.s.i. (2.76 MPa) between the tool face and the cut surface dramatically improved tool life. It was further disclosed that there was a very distinct critical pressure above which tool life actually decreased. It was hypothesized that at higher pressures the cooling liquid swept the surface too fast, reducing heat transfer.
As a result of this early work, lubricooling jets have generally been restricted to pressures below about 400 p.s.i. (2.76 MPa) and it was not expected that jets of higher pressures would be effective in lubricating or cooling the cutting zone.
Even with the presently available lubricating and cooling techniques, the temperature and pressure and friction in the cutting zone are high and thus tool life is relatively short, cutting forces are high, material removal rates are low, and surface quality is generally poor. A particular problem with the more difficult to machine materials is chip formation--the resulting chips are long and straight and ribbon-like. These chips can interfere with machining so chip breakers must be incorporated in the tool. These chip breakers increase cutting force, and are themselves subject to severe wear, which shortens tool life.